This invention relates to plasma enhanced chemical vapor deposition. More particularly, it relates to deposition in an expanding thermal plasma (hereinafter referred to as “ETP”) system to coat large areas of a substrate.
The use of ETP to deposit coatings, especially protective coatings, on substrates is known. For example, U.S. Pat. No. 6,110,544 describes a method of forming coatings on a plastic substrate such as polycarbonate. The coatings that may be deposited include silicon oxide-based hardcoats, which can protect the plastic surface from abrasion. Also capable of deposition are metal oxide-based coatings such as zinc oxide. Coatings of these types are produced by introducing an organosilicon or organometallic compound into an ETP and causing the plasma stream produced by said ETP to impinge upon the surface of the substrate. By using ETP, particularly those using equilibrium thermal plasma arc generators, high coating deposition rates may be achieved at relatively low temperatures. It is particularly desirable that the substrate temperature be maintained lower than its glass transition temperature and/or softening temperature.
A problem with ETP deposition as described in the prior art is its incapability of efficiently producing a coating over a large area of a substrate. A single ETP source typically coats an area of about 75-200 cm2. Thus, many passes of a substrate into contact with the ETP would be necessary to coat the entire surface thereof.
Various publications disclose coating apparatus and systems using a plurality of coating units. For example, U.S. Pat. No. 4,948,485 discloses the disposition of a number of cascade arc plasma torches in a circular array around an axis, which may, for example, be a wire to be coated. Such a system is incapable being used to coat a single surface of, for example, a planar substrate.
U.S. Pat. Nos. 5,302,271 and 5,441,624 disclose multi-anodic arc coating systems in which the arcs are directed at a single surface of a substrate. Anodic arcs are fundamentally different from ETP sources in that they do not generate a thermal plasma and they use a consumable anode as the source material for deposition. In addition, those arcs are used for physical vapor deposition. As such, their operating parameters, such as arc-to-arc spacing, arc to substrate distance, and deposition pressure, are not adapted to ETP deposition.
It is of interest, therefore, to develop an apparatus and method suitable for coating relatively large areas of a substrate by ETP deposition. In particular, an apparatus and method that is adaptable to various configurations of substrates and to the particular parameters of ETP systems is desirable.